The invention relates to a fixed elevation subsurface decanting device used primarily for removing a portion of liquid above the apparatus and, more particularly, for removing settled and clarified liquid in the top third of the liquid in a sequencing batch reactor (SBR) waste treatment process.
An EPA report entitled "Technology Evaluation of Sequencing Batch Reactors", by James M. Montgomery, September 1984, discloses various aspects of SBR technology and is incorporated by reference to illustrate the state of the art.
In a conventional continuous flow waste activated treatment, the effluent to the system first enters an aeration tank that contains a mixture of waste to be treated and various forms of bacteria that assimilate and liquify the solids and convert them into a non-polluting liquid. The mixture is aerated by aeration devices, such as jet aerators, and diffusers. Air is absorbed into the liquid as dissolved oxygen and is readily available to the various forms of aerobic bacteria to metabolize the pollutants. A so-called F/M ratio is controlled within a defined range for optimum treatment efficiency. "F" represents the influent food and "M" the mixed liquid suspended solids (MLSS). As liquid and solid pollutants enter the aeration tank, an equal volume of liquid and MLSS is displaced from the tank. This volume of liquid is transferred by gravity to a clarifier where the solids settle and the clarified liquid is decanted over a weir. The settled solids (sludge) are pumped from the clarifier back to the aeration tank. A portion of the sludge is discharged to waste in order to maintain the proper F/M ratio in the aeration tank.
State and federal authorities have recognized that the velocity near the overflow weir in a clarifier must be kept low in order to prevent the formation of eddy currents, in this area, that would impede the settling of the solids in the clarifier. Most States adhere to the recommendations of the so-called "Ten States Standards" for the clarifier design and the overflow rate of the weirs within the clarifier. This standard recommends that the weir length be equal to 20,000 gallons per day (gpd) per foot of weir at the peak hourly flow rate. Since the peak hourly flow rate can be two to three times the average, a one million gallon per day (mgd) plant can have an hourly peak of three million gallons per day (mgd). Therefore, the required weir length is determined by dividing 3 mgd by 20,000 gpd. In such case, 150 foot of weir is needed and provides a capacity of approximately 14 gpm per foot of weir.
In surveying many sequencing batch reactor installations, it was discovered that the main reason for failure to meet expected discharge quality can often be traced to the design of the decanting device. For a 1 mgd SBR plant, the total length of weir usually is less than 25 feet per tank. Since each tank decants a maximum of six times a day and generally for 45 minutes per decant, the flow rate required to handle 1 mgd is 74 gpm per foot of weir. This is over five times the recommended maximum overflow from a clarifier. Unless a pumped decanter or a flow regulating valve is used on the discharge from the decanter, the actual flow can be three times higher than this rate due to the fact that all decanters used today have an outlet below the low water line and the flow from the decanter is usually a function of the varying hydraulic head between the high and low water lines in the aeration tank.
U.S. Pat. No. 4,596,658 refers to a design flow of up to 300 gpm per foot of weir. This is more than 20 times the established recommended standard for overflow rates of clarifier weirs, as defined by the "Ten States Standards". It is recognized that since an SBR also functions as a clarifier with a settle cycle prior to decanting, the decanter or overflow weir should be designed to recognized good engineering practices. After settling the contents in the tank, an interface exists between the clarified liquid at the top of the tank and the MLSS nearer the bottom of the tank. The depth of the MLSS is a function of the settleability and the quantity of solids in the MLSS content. Although in a well operated plant, this settled sludge should occupy only 50 to 60% of the tank volume, it is not unusual to have the interface occurring 3 to 4 feet from the top of the tank. Whether using a floating type of decanter or fixed elevation decanter, the interface can be in close proximity to the decanter at some time during the decant cycle.
It became apparent to the inventor that a decanting device should be designed so that it would come close to the recommended maximum flow rate per foot of weir length, so that it can be fixed or floating with no moving parts within the tank, so that it excluded solids when the tank was filling or being mixed and aerated, and so that it had a constant discharge rate without expensive flow control devices. The approach velocity into the decanter should be between 0.125 and 0.25 fps.
Various decanter features are disclosed in the patent literature. For example, a surface decanter is disclosed in U.S. Pat. No. 4,290,887 which has a moving header pipe with weir slots having V-notches cut in trailing edges of the slots. A liquid separator is disclosed in U.S. Pat. No. 4,396,508 which includes inclined separator baffles that help facilitate separation of different phases in the liquid. A floating decanter apparatus is disclosed in U.S. Pat. No. 4,711,716 while a strainer device with a rotating body is disclosed in U.S. Pat. No. 4,024,062. None of these patents disclose a combination of features which would have all the advantages and requirements noted above.